Wavelength division multiplex systems require semiconductor lasers that are accurately set at the ITU (International Telecommunications Union) frequencies for proper operation. Conventional semiconductor lasers used for such systems are preset to requisite wavelengths by incorporating a distributed Bragg grating in or near the active region of the laser. The manufacturing technology is sufficiently complex for precise wavelength setting. Therefore, yields of lasers at the precise and guaranteeable ITU wavelengths are low, leading to high manufacturing costs.
Additionally, once a laser of a given wavelength is fabricated, its wavelength cannot practically be changed, except for very small, but undesirable variations, resulting from temperature or current tuning of the dielectric constant or the dimension of the grating. Moreover, as lasers age, undesirable wavelength may occur as a result of prolonged usage.
Wavelength variations, either due to aging or due to imprecise manufacturing, may degrade the performance of a system since the Dense Wavelength Division Multiplex (DWDM) wavelengths, set by ITU standards, have to propagate through multiplexers and/or demultiplexers (or other wavelength sensitive elements) which are set to precise wavelengths using passive filters or other wavelength discriminating components. Consequently, such lasers often require repair and replacement.
Additionally, a laser is needed for each wavelength used, and some operations may require as many as 60 or more lasers. Thus, there is a need for a single laser component that can be tuned to generate any of the ITU wavelengths with precision and under software control.
Additionally, optical switches will allow any wavelength channel to be switched to any of the customers. In order to make sure that the return wavelength from a particular customer is the same as the wavelength received by the customer without the customer premises being stocked with all possible ITU wavelength lasers, it would be very desirable to have a single “laser” which can be tuned to the desired ITU wavelength precisely, quickly, and under system administrator's software control.